Initial commit of a mostly-working implementation of __STDC_ISO_10646__,
with collation support using the Unicode Collation Algorithm.
The conversion from men/ku/ten form to Unicode is a gross hack at present.
Fixing this, and fleshing out the LC_COLLATE locale component, are next
on the agenda.

Import wcsnlen(3) to libc
The wcsnlen(3) function conforms to POSIX.1-2008 and is an addition to the
ISO C standard.
size_t wcsnlen(const wchar_t *s, size_t maxlen);
The wcsnlen(3) function computes the number of wide-characters in a wide-
-string to which s points, not including NULL terminating wide-character
code and checking no more than maxlen wide-characters. This function never
examines wide-characters beyond a wide-string of maxlen size.
This function is a safer version of wcslen(3):
size_t wcslen(const wchar_t *s);
Update STANDARDS section of wmemchr(3) describing wide-character functions.

sync with head.
for a reference, the tree before this commit was tagged
as yamt-pagecache-tag8.
this commit was splitted into small chunks to avoid
a limitation of cvs. ("Protocol error: too many arguments")

Retire varargs.h support. Move machine/stdarg.h logic into MI
sys/stdarg.h and expect compiler to provide proper builtins, defaulting
to the GCC interface. lint still has a special fallback.
Reduce abuse of _BSD_VA_LIST_ by defining __va_list by default and
derive va_list as required by standards.

Add a new feature-test macro, _NETBSD_SOURCE. If this is defined
by the application, all NetBSD interfaces are made visible, even
if some other feature-test macro (like _POSIX_C_SOURCE) is defined.
<sys/featuretest.h> defined _NETBSD_SOURCE if none of _ANSI_SOURCE,
_POSIX_C_SOURCE and _XOPEN_SOURCE is defined, so as to preserve
existing behaviour.
This has two major advantages:
+ Programs that require non-POSIX facilities but define _POSIX_C_SOURCE
can trivially be overruled by putting -D_NETBSD_SOURCE in their CFLAGS.
+ It makes most of the #ifs simpler, in that they're all now ORs of the
various macros, rather than having checks for (!defined(_ANSI_SOURCE) ||
!defined(_POSIX_C_SOURCE) || !defined(_XOPEN_SOURCE)) all over the place.
I've tried not to change the semantics of the headers in any case where
_NETBSD_SOURCE wasn't defined, but there were some places where the
current semantics were clearly mad, and retaining them was harder than
correcting them. In particular, I've mostly normalised things so that
_ANSI_SOURCE gets you the smallest set of stuff, then _POSIX_C_SOURCE,
_XOPEN_SOURCE and _NETBSD_SOURCE in that order.
Tested by building for vax, encouraged by thorpej, and uncontested in
tech-userlevel for a week.

add wcstoll, wcstoimax, wcstoull and wcstoumax. (lib/20656)
- these are defined in C99 (wcsto[ui]max) and SUSv3.
- these implementations are written by sigsegv@s25.xrea.com,
derived from NetBSD's wcstol/wcstoul.
- I just added some comments.